Secondary batteries typically known among electrochemical devices are devices which store external electric energy by converting it into the form of chemical energy and then generate electricity when needed. The secondary battery is referred to as a “rechargeable battery”, meaning that it can be recharged many times, and provides both economic advantage and environmental advantage in comparison to a primary battery which is discarded once used. A lead-acid battery, a nickel-cadmium (NiCd) battery, a nickel metal hydride (NiMH) battery, a lithium (Li)-ion battery, and a Li-ion polymer battery are known as the secondary battery.
Weight reductions in portable devices, such as notebooks and mobile phones, or automotive parts have been required as wireless communication technology has gradually developed. As a result, demand for secondary batteries that are used as energy sources of these devices has been increased.
Secondary batteries are prepared by putting an electrode assembly composed of an anode, a cathode, and a separator into a cylindrical or prismatic metal can or a pouch-type case formed of an aluminum laminate sheet, and then injecting an electrolyte thereinto. Since secondary batteries are mostly formed in a cylindrical-type, prismatic-type, or pouch-type structure, a predetermined space for installing these secondary batteries is essentially required. Thus, the development of portable devices may be restricted.
Recently, an easily deformable new type of secondary battery has been required, and a linear battery, in which a ratio of a length to a cross-sectional diameter is very large, such as a cable-type battery, has been suggested in order to meet the requirement.
Specifically, the cable-type secondary battery is composed of an inner electrode having a predetermined-shaped horizontal cross section and extending in a longitudinal direction; an electrolyte layer formed on an outer surface of the inner electrode and serving as an ion path; an outer electrode formed by surrounding the inner electrode and the electrolyte layer; and a protective coating layer disposed around the outer electrode. Since the cable-type secondary battery has flexibility as well as a linear structure, the cable-type secondary battery may be freely deformed. Also, the cable-type secondary battery may have a high battery rate because the plurality of inner electrodes is included in a pipe-type outer electrode. Furthermore, since a capacity balance between the inner electrode and the outer electrode may be easily controlled by adjusting the number of inner electrodes and the electrolyte layer is formed in the inner electrode, a short between the electrodes may be prevented.
However, since an electrolyte in a liquid state is mainly used in the cable-type battery, the possibility of volatilization of an organic solvent may not only be high but combustion and electrolyte solution leakage due to the increases in ambient temperature and temperature of the battery itself may also occur. Thus, the stability of the battery may be poor. In a case where a polymer electrolyte is used as an electrolyte solution in order to improve the above limitations, the inflow of the electrolyte to an electrode active material may be difficult, and thus, resistance of the battery may increase and capacity characteristics and cycle characteristics may degrade.
Therefore, the development of a newly configured cable-type battery having an electrolyte injected thereinto, which may improve a battery performance by preventing the leakage of a typical electrolyte and forming a stable interface between the electrode and the electrolyte, is urgent.